The gas occurrence in the coal seams in China has characteristics including micro-porosity, low-permeability, and high-absorptivity. The average permeability of the coal seams is 1.1974×10−18 to 1.1596×10−14m2, and the gas pre-extraction rate is very low, having a strong impact on the safe production in coal mines.
At present, hydraulic measures, represented by hydraulic slotting and hydraulic flushing, etc., have been widely applied in the gas control process in the coal mining fields in China, owing to their efficient pressure relief and permeability improvement effect. However, owing to the fact that the geologic conditions of the coal seams in China are complicated and the permeability of the coal seams is low, if only a single hydraulic measure is used, because of the limited pressure relief and permeability improvement effect incurred by the limited fracturing ability of water-jet cutting and high-pressure water impact, the gas extraction concentration will be low, the extraction cycle will be long, and the requirement for intensive coal mining cannot be met.
In addition, relevant research findings have indicated that the pore structures of coal masses in China are mainly micro-porous structures in which a large quantity of gas adsorption spaces are formed and the gas absorptivity of the coal mass is very strong, resulting in high gas content in the coal mass and difficult gas desorption. Consequently, it is difficult to extract the gasses from the coal seams, the flow rate of gas extraction from a single borehole is quickly attenuated, and the resulting extraction is poor. Available research findings have demonstrated that the gas desorption from coal mass can be promoted and the quantity of gas extraction can be increased as the temperature of the coal mass increases.